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1. A hollow iron pipe is 21 cm long and its external diameter is 8 cm. If the thickness of the pipe is 1 cm and iron weights 8 g/cm³, then the weight of the pipe is:

A. 3.6 kg

B. 3.696 kg

C. 36 kg

D. 36.9 kg

Solution:

\begin{aligned} External radius = 4 c m \\ Internal radius = 3 c m \\ Volume of iron \\ = (\frac{22}{7} \times [{(4)}^{2} - {(3)}^{2}] \times 21) c m^{3} \\ = (\frac{22}{7} \times 7 \times 1 \times 21) c m^{3} \\ = 462 c m^{3} \\ ∴ Weight of iron = (462 \times 8) g m \\ = 3696 g m \\ = 3.696 k g \end{aligned}

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